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DEPARTMENT OF COMMERCE 

BUREAU OF STANDARDS 

S. W. STRATTON, Director 



TECHNOLOGIC PAPERS OF THE BUREAU OF STANDARDS, No. 84 



(Issued November 17, 1916] 



FAILURE OF BRASS. 3.— INITIAL STRESS PRODUCED BY THE 
•'BURNING IN" OF MANGANESE BRONZE 



By Paul D. Merica, Associate Physicist, and C. P. Karr, Associate Physicist 

r I 

' ^ ^ / fo &«.. INTRODUCTION 

The present investigation has been made in connection with the 
failure of a number of manganese bronze valve castings in the 
Catskill Aqueduct (New York) , f ailm-es which are described briefly 
by A. D. Flinn, of the New York Board of Water Supply, as 
follows : ^ 

* * * Until the spring of 1915 castings were believed to be immune from troubles, 
excepting those incident to foundry work and of the kinds which might occur in any 
metal. Since then, however, a number of castings from at least three different found- 
ries producing manganese bronze have been found cracked. All of these castings, 
before acceptance, had been subjected to hydrostatic test pressures of 200 or 300 
pounds per square inch, for a half hour or more, and, of course, appeared to be satis- 
factory at the time of preliminary acceptance. Some months later, after having been 
placed in the structures, some of these castings leaked under pressures of only a few 
pounds. In some castings the cracks have grown longer as time has elapsed, and in 
some, with the passage of time, additional cracks have been discovered. These cracks 
are fine, and often difficult to detect on the surface until the casting is put under 
hydrostatic pressure. In most cases, indeed, in nearly all, these cracks appeared to 
be close to or in a repair made by the method of burning in. * * * 

The castings referred to above are quite large, up to 22 000 
pounds in weight, such that neither preheating of the casting for 
the welding or burning in of defective areas, nor subsequent anneal- 
ing of the whole casting could be conveniently carried out. As 
a result of the local heating of welding and consequent unequal 
contraction of different constrained parts of the casting, stresses 
remained in the casting, particularly severe near and within the 

'A. D. Fliun, Some Experiences with Brass in Civil Engineering Works, Trans. Amer. Inst. Metals, 
6; 191s- 

57030°— 16 



fl 



Ponograph 



2 Technologic Papers of the Bureau of Standards 

bumed-in areas; these stresses were in all probability responsible 
for the subsequent failure at these points. 

The determination of the values of such stresses, in the case of 
castings of manganese bronze, in correlation with the physical 
properties and structure of this material as welded or burned in 
seemed desirable. 

THE MEASUREMENT AND CALCULATION OF STRESS 

For the investigation of these stresses a form of casting, shown 
in Figs. I and 2, was chosen. It is a double-bar frame, the two 
crossbars, A and B, having sections of area i by i inch and 3 by 

3 inches, respectively, and being connected by heavy and stiff 
ends, M and A'^. The inside length from a to & (Fig. i) of the bars 
was about 9 inches. 

A portion of the bar A, varying from >^ to 2 inches in length, 
was removed by sawing and replaced by burning in with the same 
material, care being taken to keep the bar, B, cool during the 




Fig. 2. — Form of casting used in hurning-in experiments 



operation. The pouring gate for the burning in was then sawed 
off, gauge points placed just inside of a and h, and readings taken 
with a strain gauge before and after sawing through A , outside of 
the gauge point, at either a or &. Knowing the elastic modulus 
of the material and the contraction over the 8-inch gauge length, 
a direct calculation gives the value of the stress in the bar A . 

Some interest attaches to the method by which the welding 
was accomplished. In all cases the casting was placed on its side 
and embedded in green sand, a pit was then hollowed out by hand 
around the severed ends of A, such as to expose these ends, and 
the bar from ^ to i inch back, and to allow a small channel way, 
from rs to yi inch wide around the bar at these ends. The sand 
was here built up slightly and a hollow iron cylinder, about 4 
inches high, set over this pit, thus providing a sufficient pouring 
head. It was found necessary to preheat the ends to be welded, 
not only by means of a torch but also by making a small reservoir 
in the sand directly under the burning-in pit, separated from it 



D. of D. 
DEC ^ f9,ff 






Bureau of Standards Technologic Paper No. 84 




Fig. I. — Casting used in the burning-in experinieiits 




Fig. 3. — Grain structure of section through burned-in zone. Etched with 
ammoniacal copper chloride. X-T 




Fig. 4. — Within bumed-in zone 



Fig. s- — Outside of bumed-in zone 



Figs. 4 and 5. — Microstructure of burned-in bronze casting. Etched with ammonium 

hydroxide, y^ioo 



"Burning In" of Manganese Bronze 



by sheet-iron plates, and pouring hot metal into this reservoir 
immediately before burning in. No flux of any kind was used. 

The molten metal was first poured into this reservoir, then into 
the pit, completely enveloping the ends and filling the pouring 
head. 

In order to be certain that the bar, B, of the casting was actually 
kept cool during the operation, this part was in several instances 
water-cooled while the weld was made. This was done by boring 
a I -inch hole longitudinally through the center of B, attaching 
pipe and hose and running water through during the operation 
of burning in. 

The material used was that of a large scrapped manganese 
bronze casting, kindly furnished by the New York Board of Water 
Supply, of the following composition: 



Per cent. 

Copper 58. 5 

Zinc 39- I 

Tin I. o 



Per cent. 

Lead Trace. 

Iron 1.4 

Mansranese None. 



This method gave uniformly sound welds, as will be seen from 

Figs. 3. 

TABLE 1 

Stresses Produced by the Burning In of Manganese Bronze 



Casting 


Length of 
bar A, re- 
moved and 
burned in 


Tensile stress meas- 
ured after burning in 




Lbs./in.2 


Kg/ cms 


232a 


Inch 

0.25 
.50 

.50 

.50 

.50 

2.0 

2.0 


9600 

9200 
8500 
8500 
6400 
• 8500 


680 


232b ' 


(°) 


232c 


640 


232d 


600 


232e 3 


600 


232f 


450 


232e'- 


600 







2 Weld not sound. 

3 This casting was locally annealed as described below before the stress determination was made. 
* In this case the part B of the casting was cooled with running water as described above. 

Table i gives the values of the tensional stresses found in the 
bar A of the various castings tested. 

It is now not difficult to make an approximate calculation of 
the tensile stress in the bar A, which should result from the 
burning in. 



4 Technologic Papers of the Bureau of Standards 

Let E = the modulus of elasticity of the material (in tension) , 
E' =/, (E, geometry of object to be burned in) =the recip- 
rocal of the fractional elongation per unit stress, 
between a and b, of the frame, M B N 
I =the distance from a along A toward b 
L =the distance a b 

S = the resultant unit stress along A , due to burning in. 
At the moment of burning in — ^i. e., of the soUdification of the 
burned-in metal — the temperature distribution along A is given 
by 

(1) t = ct>il) 
and 

(2) l = \l/(t), gives the equation of linear expanson of the material. 

Then in cooling down to room temperature, 20° C, a change of 
length will occur between a and b, equal to 

AL = — (amount of thermal contraction) + (elastic 
elongation due to the constraint by the 
bar 5) 
also 

= — (elastic contraction due to the constraint 
by A) 



-H 



t=<p(.[) 

dl,.LS 
dt^^-^^ 



__LS 
~ E' 
Therefore, 

(3) -97=^- I dl I j^dt 



I dl I yP' 



(4) -^=¥- I ^M ^'W^^ 



The assumption has been made that at every moment during cool- 
ing of the bar A the resulting stresses have been below the elastic 
limit of the material. The equation (4) gives the general case 
and can not be solved unless ^(/) and ^{t) are known. Assuming 
that the coefficient of linear expansion is constant — i. e., does not 
vary with the temperature — and equal to a, and that a length of 



"Burning In" of Manganese Bronze 5 

bar A, equal to d, was, at the moment of solidification of the 
metal, at the melting point of the metal, about 900° C, the equa- 
tion (4) reduces to 

L|-88oacf = -L-|. 
h, h, 

An approximate calculation of the stiffness of the frame M B N — 
i. e., the heavy bar and ends — gives 



E' \E^9E) 



(considering the ends M and A^ as beams) 
Therefore, 

yg2oE ad 
ioL + 9 
Assuming that 

E = 15 X 10^ potuids per square inch, 
a = 0,00002 
. L = 9 inches. 
d=i.s inches. 
5 = 36 000 pounds per square inch. 

One should expect, then, a tensile stress of about 36 000 pounds 
per square inch for every inch of metal burned in in these tests. 
There results, actually, a stress of from 8000 to 10 000 pounds 
per square inch, irrespective of whether X inch or 2 inches of the 
bar A were burned in. This indicates, of course, that at some 
temperature the resulting stress becomes greater than the elastic 
limit of the material, and the material yields thenceforth, the stress 
following (with probably some time lag) the elastic limit of the 
material, such that the stress, as measured, one or two days after 
burning in, represents the true elastic limit of the material. 

PHYSICAL PROPERTIES AND MICRO STRUCTURE OF BURNED-IN BRASS 

A tensile test was carried out on a specimen from the bar A of 
No. 232g, and gave the following results: 

Ultimate strength 60 000 Ibs./in.^ (4220 kg/cm^) 

Proportional limit 13 000 lbs. /in. ^ (920 kg/cm^) 

Modulus of elasticity 15.4X10^ lbs. /in. ^ (i.iXio® kg/cm-) 

Elongation in 5 inches 19.4 per cent. 

Reduction of area 26.0 per cent. 

The fractiu-e occurred outside of the welded area, but not at 
the juncture of the original and the welded-in portions, and the 



6 Technologic Papers of the Bureau of Standards 

welded-in section of the test piece was harder and less elongated 
than the original material. The fracture showed only a few 
minute flaws ; it may be accepted, therefore, that the true elastic 
limit of the material was somewhat below the proportional limit in 
this case. 

Consideration of the microstructure of the burned-in metal 
affords an explanation for its greater hardness. Fig. 3 shows the 
grain structure within and without the welded-in area of the 
specimen 232c. It is seen that the central welded portion is of 
much finer grain than the original material, as cast, on either side, 
and the transition from welded to original structure is very abrupt. 
In Figs. 4 and 5 are shown the microstructures, inside and outside, 
respectively, of the bm-ned-in zone. The structures are those of 
normal manganese -bronze, there being no difference in the two 
other than that of size of grain. No evidence of overheating or of 
bm*ning out of the zinc near the juncture of the zones was noted. 
These results may be compared instructively with those obtained 
by CarnevaH,^ on similar alloys, in which the welding was done by 
means of the oxy-acetylene flame. In this case a burning out of 
the zinc took place at the juncture of the weld. It is not surprising 
to find such differences in the results by the two methods, as greater 
opportunity for oxidation and overheating is given by the use of 
the oxy-acetylene process. 

The bar A of casting 232h, with 2 inches burned in, was brushed 
over continually with a solution of mercmrous nitrate for about 
two weeks, but showed at the end of that time no fissures or sign 
of failure. 

A small portion, about 6 inches, of the bar A of the casting 
23 2e was woimd with Nichrome wire and the bar A thus locally 
annealed at a temperature of 300° C for about one hour. The 
stress in the part A remained unchanged by this treatment. It 
may be noted that if it is wished to relieve the stresses in A by 
some "heat treatment" affecting it only, the part B being kept 
at the ordinary temperature, the part A should be cooled 50° or 
100° instead of being heated. 

CONCLUSIONS 

The experiments described have indicated how readily severe 
stresses may be introduced into a casting by the burning in or 
welding of a "constrained" area or portion of it. The form of 

^ F. Camevali, Autogeneous Welding of Copper and its Alloys, J. Inst. Metals, 8, p. 282; 1912. 



"Burning In" of Manganese Bronze 7 

casting used in these experiments was such that no bending or 
distortion, tending to relieve the stresses, was possible during 
cooUng, and this must be borne in mind in applying these results 
to the consideration of the effect of burning in of more complicated 
shapes, where such distortion does occur. 

In the spherical shell or dome-shaped valve castings of the New 
York Board of Water Supply, for instance, burned in would tend 
to flatten the shell, and in so doing partially reHeve these stresses, 
and it is most difiicult to calculate the stresses in such a case. 
The authors are inclined to believe that even in these cases local 
stresses of values equal to the true elastic limit must have been 
produced and which would account for subsequent failure. 

The conclusions to be drawn therefore are : 

1 . That the welding in of constrained portions of castings (forg- 
ings, wrought articles, etc., naturally, as well) of manganese 
bronze, produces, in general, local initial tensional stresses within 
and near the biurned-in zone, of value equal to the true elastic 
limit of the material unless the shape of the casting is such that 
extensive distortion may occur. 

2. That such castings should therefore be either preheated care- 
fully for welding, such that all parts of the casting cool down 
together from a dull red heat or the casting should be subsequently 
annealed. Experience * indicates that a low temperature ajineal 
is sufficient for this purpose — e. g., from 400 to 500° C (760° to 
940° F) — for from one to two hours. Either of these precautions 
should eliminate these local stresses resulting otherwise from the 
burning in and should produce castings free from danger of sub- 
sequent cracking. 

The authors wish to express their appreciation of helpful sug- 
gestions received in conference and correspondence with Messrs. 
A. D. Flinn, S. W. Miller, and others. 

Washington, July 12, 1916. 

^ p. D. Merica and R. W. Woodward, Failure of Brass, i. — Microstructure and Initial Stresses in 
Brasses of the Type 60 per cent Copper and 40 per cent Zinc, B. S. Tech. Paper No. 82. 



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